You need to obtain the wiring diagram for the relay for this horn used on the 12GS. I've had mine on there for several years and it works perfectly well without being shut down by the Canbus.

The wiring however, is not how you'd normally wire a horn using a relay. I don't have the schematic handy, so I can't tell you how to wire it so it works, but there is a way of doing it. I do recall there are 2 leads to the battery, 1 to the bike's +ve horn harness..

Gadget Boy, there are actually multiple wires not depicted in the diagram. I pull the horn "signal" from the oem horn to the relay. When I push the horn button on the handlebar, that signal tells the horn relay to send power from the AP1 to the Stebel and then the stebel fires. That actually worked. But in the process of drawing power from the AP1, the BMW Aux Plug that feeds the 2nd relay (the one that tells the battery to send power to the AP1) shuts down.

Now why would drawing power from the battery--through the AP1--to the stebel, cause the CANBUS to shut down. The horn should not put any additional load on the CANBUS system at all.

The stebel intersects the CANBUS in two places: at the OEM horn and at the Aux Plug. In both cases, the connection is to a relay and that is wired to post 85 on the relay which should be milliamps of current. The only thing that current is for is to tell the switch to close and send power from the source to the device.

Source= battery Device= AP1
Source=AP1 Device= Stebel

I need ideas on how to isolate the problem. My next step is to wire the horn to the battery. That will remove one of the relays and one of the CANBUS connections. That won't however tell me why it is doing this in the first place.